肥胖 is a clinical/neutral term for obesity, commonly used in medical contexts. Calling someone 肥胖 directly is blunt. In casual speech, 胖 (pàng) alone is softer, while 肥 (féi) alone can sound rude when applied to people.
mistakes
肥 is typically used for animals or meat (肥猪, fat pig; 肥肉, fatty meat). Using 肥 alone for people sounds impolite. The compound 肥胖 is acceptable in formal/medical contexts.
